In accordance with the trend of the present times toward promotion of the health, production of distilled liquors, especially Japanese distilled spirits (shochu) has been increasing. The distilled spirit, shochu, is produced by subjecting a raw material such as sweet potato, rice, barley or the like to alcoholic fermentation, and separating a fraction in distillation. In the step of separation, large amounts of distillation residues (hereinafter referred to as distillation waste) are generated. Also, in the manufacturing process of bioethanol for fuel, which mainly uses various starchy materials such as corn, sugar cane and the like as the raw materials, considerable distillation waste is generated similarly.
The distillation process based on alcoholic fermentation of raw materials such as the above-mentioned starchy materials has caused the problems in the treatment of considerable amounts of distillation waste generated. Conventionally, the distillation waste has been mostly discarded by dumping in the ocean, returning to the earth, incineration and the like. However, dumping of wastes into ocean is controlled under the “London Convention” and supposed to be completely prohibited. The method of returning the waste to the earth has produces the problems of pollution of groundwater and rivers; and the method of incineration has a problem in cost. In light of this, as an alternative to the conventional discarding methods, the establishment of a method for making effective use of the distillation waste, for example, as livestock feed, is considered to be a high priority.
Recycling of enormous amounts of the distillation waste as a solid feed, manure or the like can be realized if the distillation waste can be subjected to effective solid-liquid separation. Current methods for the solid-liquid separation include floatation, centrifugal separation, coagulation settling and the like. The water content of the distillation waste is usually 90 mass % or more and the viscosity thereof is high, so that the solid-liquid separation cannot be completely achieved by any of the above-mentioned current methods, or the separation is very difficult at the present stage. There is known a dewatering method for sludge using a cationic polymer coagulant and an anionic polymer coagulant (Japanese Patent Examined Publication (JP Kokoku) Hei 2-11320). Even though the process for the above-mentioned dewatering method is applied to the distillation waste of starchy materials, good results cannot be obtained.